let a = [1, 2, 3, , 4];
console.log(a.fill(0)); // [0,0,0,0,0]
console.log(a); // [0,0,0,0,0]
console.log(a.fill(9, 2)); // [0,0,9,9,9]
console.log(a.fill(8, 1, -2)); // [0,8,8,9,9]
console.log(a.fill(7, 2, 4)); // [0,8,7,7,9]
